1. Field of the Invention
The invention relates to an apparatus for writing and/or reading information in substantially concentric tracks on a disc-shaped record carrier, which apparatus comprises:
a light source for generating a light beam,
an objective lens system for focussing the light beam to form a light spot on the record carrier,
first detection means which are responsive to the track structure to generate a first control signal which varies substantially proportionally to the radial displacement of the light spot relative to said track over a control range of p/4 to either side of the track, p being the track pitch, and
radial positioning means for controlling the radial position of the light spot on a track under the control of said first control signal.
2. Description of the Related Art
Such an apparatus is known from the published British Patent Application No. 2,073,452.
It is to be noted that such apparatus is suitable for different uses. The best-known uses are those where disc-shaped record carriers containing a non-erasable program can be read by means of an optical read unit. For this reference is made to the Compact Disc Digital Audio System (CD audio) and derivatives of this system, such as CD-ROM (for data storage) and CD-I (for interactive purposes).
However, the apparatus may also be used in systems enabling information to be both recorded in and read from a disc-shaped record carrier. Examples of such systems are systems in which information can be inscribed in a metal layer by locally evaporating this metal layer by means of a light beam, systems in which a crystalline material is converted into an amorphous material or vice versa by means of a light beam, and magneto-optical systems.
In systems with a write capability it is essential that the blank record carrier already be provided with a control track, for example in the form of a real or imaginary track structure. An example of this is a prestructured track in the form of a pregroove. An example of an imaginary control track structure is a track which is defined by pits (servobytes) which are situated at regular distances in the longitudinal direction of the track and which are slightly shifted relative to this track in a radial direction. Radial position control of the light spot during recording is then effected by means of such control track.
During reading of information reading radial control can be achieved by utilising the tracks which contain the recorded information.
The apparatus known from said British Patent Application is found to be sensitive to mechanical shocks, which may sometimes give rise to loss of track. Loss of track means that during recording or reading the light spot has moved off the track so far that the information can no longer be recorded or read correctly.